Download Atomic Number and Mass Number

The Structure of
the Atom 4.2
Properties of Subatomic Particles
The three subatomic particles are:
Protons, electrons, and neutrons.
Properties of Subatomic Particles
Protons
Based on experiments with elements other than
gold, Rutherford concluded that the amount of
positive charge varies among elements.
A proton is a positively charged subatomic
particle that is found in the nucleus of an atom.
Each proton is assigned a charge of 1+.
Each nucleus must contain at least one proton.
Properties of Subatomic Particles
Electrons
The particles that Thomson detected were later
named electrons.
An electron is a negatively charged subatomic
particle that is found in the space outside the
nucleus.
Each electron has a charge of 1.
Properties of Subatomic Particles
Neutrons
In 1932, the English physicist James Chadwick
carried out an experiment to show that neutrons
exist. Chadwick concluded that the particles he
produced were neutral because a charged object
did not deflect their paths.
A neutron is a neutral subatomic particle that is
found in the nucleus of an atom.
It has a mass almost exactly equal to that of a
proton.
Comparing Subatomic Particles
Protons, electrons, and neutrons can be
distinguished by mass, charge, and location
in an atom.
Comparing Subatomic Particles
Everything scientists know about subatomic particles is
based on how the particles behave in experiments.
Scientists still do not have an instrument that can show
the inside of an atom.
Comparing Subatomic Particles
Similarities and differences between protons, electrons,
and neutrons.
• Protons and neutrons have almost the same mass.
• About 2000 electrons equal the mass of one proton.
• An electron has a charge that is equal in size to, but the opposite
of, the charge of a proton.
• Neutrons have no charge.
• Protons and neutrons are found in the nucleus.
• Electrons are found in the space outside the nucleus.
Atomic Number and Mass Number
Atoms of different elements have different
numbers of protons.
Atomic Number and Mass Number
Atomic Number
• The atomic number of an element is the
number of protons in an atom of that element.
• All atoms of any given element have the same
atomic number.
• Each hydrogen atom has one proton in its nucleus.
• Hydrogen is assigned the atomic number 1.
• Each element has a unique atomic number.
Atomic Number and Mass Number
Each element has a different atomic number. A
The atomic number of sulfur (S) is 16.
B The atomic number of iron (Fe) is 26.
C The atomic number of silver (Ag) is 47.
Atomic Number and Mass Number
Atoms are neutral, so each positive charge in an
atom is balanced by a negative charge. That
means the atomic number of an element also
equals the number of electrons in an atom of that
element.
• Hydrogen has an atomic number of 1, so a hydrogen
atom has 1 electron.
• Sulfur has an atomic number of 16, so a sulfur atom
has 16 electrons.
Atomic Number and Mass Number
Mass Number
The mass number of an atom is the sum of the
protons and neutrons in the nucleus of that atom.
To find the number of neutrons in an atom, you
need the mass number of the atom and its atomic
number.
The atomic number of aluminum is 13. An atom
of aluminum that has a mass number of 27 has 13
protons and 14 neutrons
Isotopes
Isotopes of an element have the same
atomic number but different mass numbers
because they have different numbers of
neutrons.
Isotopes
Isotopes are atoms of the same element that have
different numbers of neutrons and different mass
numbers.
To distinguish one isotope from another, the
isotopes are referred by their mass numbers. For
example, oxygen has 3 isotopes:
oxygen-16, oxygen-17, and oxygen-18.
All three oxygen isotopes can react with hydrogen
to form water or combine with iron to form rust.
Isotopes
With most elements, it is hard to notice any
differences in the physical or chemical properties of
their isotopes.
Hydrogen is an exception.
Hydrogen-1 has no neutrons. (Almost all hydrogen
is hydrogen-1.) Hydrogen-2 has one neutron, and
hydrogen-3 has two neutrons. Because a hydrogen1 atom has only one proton, adding a neutron
doubles its mass.
Isotopes
Water that contains hydrogen-2 atoms in place of
hydrogen-1 atoms is called heavy water. Hydrogen-2
atoms have twice the mass of hydrogen-1 atoms, so the
properties of heavy water are different from the properties
of ordinary water.
Assessment Questions
1. In which way do isotopes of an element differ?
a.
b.
c.
d.
number of electrons in the atom
number of protons in the atom
number of neutrons in the atom
net charge of the atom
Assessment Questions
1. In which way do isotopes of an element differ?
a.
b.
c.
d.
number of electrons in the atom
number of protons in the atom
number of neutrons in the atom
net charge of the atom
ANS: C
Assessment Questions
1. Of the three subatomic particles that form the atom, the
one with the smallest mass is the neutron.
True
False
Assessment Questions
1. Of the three subatomic particles that form the atom, the
one with the smallest mass is the neutron.
True
False
ANS:
F, electron